The present invention relates to novel electrolytes for electrochemical double layer capacitors 9EDLC) comprising doubly charged cations of N,N-dialkyl-1,4-diazabicyclo[2.2.2]octanediium (this compound is referred to as DADACO in the remaining description and claims). More particularly, the present invention relates to novel electrolytes comprising at least one salt such as DADACO tetrafluoroborate or hexafluorophosphate dissolved in aprotic polar solvent or mixture of such solvents, a process for their preparation and their use in EDLC.
Organic electrolytes based on polar aprotic solvents and tetraalkylammonium perchlorates, tetrafluoroborates or hexafluorophosphates are widely used for EDLCs (JP Patents Nos. 49-68254, 1974; 5240025, 1977; 54-9704, 1979; 61-203628, 1986; 61-204927, 1986). They have a notable advantage over the aqueous electrolytes due to their much higher electrochemical stability resulting in higher working voltage. It will be recalled that the energy stored in the capacitor is proportional to its voltage squared:
E=0.5CU2.xe2x80x83xe2x80x83(1)
Another advantage of choosing polar aprotic solvents is a wider temperature range since the organic solvents, which are usually used in EDLCs, are liquid within a range of xe2x88x9260xc2x0 C. to +240xc2x0 C. However, the solubility of the tetraethylammonium salts mentioned above decreases rapidly with temperature so that, in practice, the lower temperature limit for the use of such electrolytes is usually xe2x88x9220 to xe2x88x9225xc2x0 C.
In WO95/20231 and Pat RU 2022926 (priority Apr. 26, 1993, publ. Nov. 15, 1994) the present inventors disclosed new tetrafluoroborates and hexafluorophosphates of dialkylamino-phosphonium derivatives to be used as supporting electrolytes in EDLCs. Said amino-phosphonium salts exhibited high solubility in polar aprotic solvents down to xe2x88x9260xc2x0 C. and smooth temperature dependence of the characteristics of a capacitor mockup.
Besides the great number of salts having a singly charged cation, such as different ammonium, phosphonium, pyridinium. imidazolium and other salts, which have been used in SC electrolytes. salts with doubly and triply charged cations have also been disclosed. Among them are flat aromatic heterocycles: pyridazine (I), pyrimidine (II), pyrazine (III) and also hexahydrotriazine (IV) alkyl derivatives (JP Patents Nos. 4-233210 and 4-233211, 1992) as shown below: 
However, as experiments performed by the present inventors have shown, these cations are not electrochemically stable enough so that the organic electrolytes based on their salts cannot be used at voltages higher than 2.5-2.8 V. On the other hand, the idea of using doubly charged cations in the electrolytes for EDLC applications looks very attractive since an increase in the ion charge density would lead to a corresponding increase in the density of a counter-charge induced in an electrode, and as a result, the double layer capacitance would also increase.
It is an object of the present invention to provide new electrolytes which contain electrochemically and chemically stable doubly charged cations and provide good performance (first of all, higher capacitance and higher working voltage) for EDLC.
It is another object of the present invention to provide new compounds useful as electrolytes for EDLCs and which can easily be synthesized using commercially produced starting materials and minimizing the formation of toxic and environmentally hazardous by-products in course of their preparation and use.
To accomplish these and other objects the present invention provides novel electrolytes according to the attached claims. These are electrolytes for EDLC comprising doubly charged bicyclic cations of N,N-dialkyl-1,4-diazabicyclo[2.2.2]octanediium (DADACO): 
where R is an alkyl C1-C4, and Y is a BF4xe2x88x92 or PF6xe2x88x92 anion.
The DADACO cations according to the present invention surprisingly combine both the high chemical and electrochemical stability of bicyclic systems, besides, the 1,4-diazabicyclo[2.2.2]octane (DABCO) is a commercially available starting compound for synthesis of new doubly charged bicyclic cations.
It was surprisingly shown that various DADACO derivatives (R=alkyl C1-C4) can be easily synthesized from DABCO by treating it with a corresponding alkyl bromide followed by substituting the bromide anion with tetrafluoroborate or hexafluorophosphate. Some of the data obtained for DADACO electrolytes are shown in FIGS. 1-3 as well as in Tables 1 and 2. Since the ethyl derivative DEDACO2+(BF4xe2x88x92)2 has demonstrated the best solubility, conductivity and stability in the experiments performed by the present inventors, this salt was also tested in SC mockupsxe2x80x94see in Examples 7-11 for how they were made, and in FIG. 4 and Table 2 for the results.